Open heart surgery is sometimes used to clear stenosed artery segments which are in close proximity to the heart. It is not uncommon during this procedure for surgeons to literally feel arterial segments with their fingers to locate hard segments (a process known as "palpation"), and in that way establish and localize a stenotic segment of the artery. While well-trained and experienced surgeons are typically adept at this task, the potential for uncertainty and error exists.
Recently, some hospitals have begun to develop direct contact scanners that use ultrasound to image areas of the body. Scanners of this type are typically used for non-invasive procedures, for example, in obstetrics. In operation, these devices direct ultrasound into the body, with various body tissues producing ultrasound echoes which are detected by the scanner and electronically used to construct an image. These scanners have proven very useful in obtaining images of certain internal body tissues, though their resolution of deep or intricate tissues is limited, and they are not readily applied to invasive situations.
A typical scanner 11 is seen in FIG. 1. The scanner includes a main scanner body 13, and a radome 15 that directly contacts an object 17 to be scanned (the human body). The radome 15 also houses a coupling fluid 19 which is used to help transmit ultrasound. As used herein, "radome" is a surface that is transparent to the imaging waves used to scan the object 17, such as an acoustically transparent window. The coupling fluid 19 is necessary in the case of ultrasound, because it permits scanning movement of an ultrasound transducer 21, as indicated by reference arrow 23, and because high frequency ultrasound does not transmit well in air.
Within the radome 15 and the coupling fluid 19, the scanner 11 includes a transducer assembly 27, consisting substantially of the transducer 21 (a directional ultrasound transducer operating in the range of 2.5 to 10.0 megahertz) and a pivotal support 29 for the transducer. The pivotal support enables the transducer to be pivoted such that the direction of produced ultrasound sweeps through a sector, as indicated by reference arrow 31, causing ultrasound to image a plane or section of object 17 to be scanned. The transducer 21 has insulated electric leads 32 which are connected to processing circuits (not shown) in the main scanner body 13. These processing circuits control the transducer to both produce ultrasound in discrete bursts, and also to detect ultrasound echoes and responsively generate an image. Typically, the leads 32 supply an excitation signal to the transducer which is on the order of 100 volts. An angle encoder 33 in the main scanner body generates a sync signal that informs the main processing unit as to the beginning of a new image frame. The transducer is moved by a reciprocating motor 35, located in the main scanner body, which pulls a belt 37 back-and-forth to pivot the transducer through the sector. The belt wraps around a pulley 39 of the transducer and is, in turn, anchored to the main body by a spring 41 and a fixed support 43.
While generally useful for non-invasive applications, such as obstetrics, cardiology and the like, direct contact scanners of the type just described have a number of shortcomings. In particular, these shortcomings make it impractical to use the scanners in invasive surgical procedures, for example, open heart surgery, or in a wide variety of other applications.
First, the transducer and its electrical leads are typically located within the coupling fluid, in order that ultrasound can be directly coupled to the object to be scanned while the transducer is being pivoted. However, this construction generally requires the use of electric potentials immediately adjacent to the radome, in close proximity to body tissues, which presents a danger of electrical leakage during surgery. This danger is particular acute if the scanner will be used near highly sensitive tissues, for example, the heart or brain.
Second, the size of the probe required to house the transducer and pivotal mounting in close proximity to the radome makes a direct contact area of the probe excessively large, rendering it difficult to use the probe in hard-to-access areas within the body cavity during surgery. For example, during brain surgery, it might be desirable to use a direct contact scanner through a bore hole in the skull to image a tumor; the typical scanner just described presents a direct contact area which is generally too large to be usable in these situations. This difficulty renders the scanners unusable for many invasive applications, as well as for most non-medical applications where quarters are cramped.
A third, related problem, is that the frequencies of ultrasound producible by the scanner just described are limited; since frequency of ultrasound produced is inversely proportional to transducer thickness (transducer material generally must be about one-half wavelength thick, given the desired frequency's speed of travel in the transducer material), high frequency transducers are relatively thin and more prone to damage where a moving transducer assembly is utilized. Generally, use of a moving transducer assembly requires use of a thick solid backing for high frequency transducers, which unfortunately imposes undesired weighting and high inertia considerations at the direct contact end of the scanner. This arrangement is undesirable, and it in practice limits the range of ultrasound frequencies that are produced by the scanner. In turn, limitation in the range of ultrasound frequencies places a limitation on the resolution that can be achieved with the scanner. To be able to properly diagnose the nature of a tumor or an occlusion in a blood vessel such as a coronary artery, it would be extremely useful to be able to characterize these tissues or lesions in extreme detail, which is generally achievable using ultrasound frequencies in the range of thirty- to fifty-megahertz, and perhaps higher.
There is a dire need for a method or device for safely imaging body tissues, particularly during surgery, which does not mandate reliance upon a surgeon and which does not expose a patient to leakage currents. Such a method or device should require only a small contact area such that it is usable in remote areas, for example, in body tissue areas such as the brain that are not easily accessed. Preferably, such a method or device should offer a precise, high-resolution imaging procedure, to enable quick diagnosis of maladies with a high degree of accuracy. Also, it would serve the physician well if the operating frequency of a scanner could be changed while the scanning is in progress. Finally, because of the requirement of disposableness due to fear of contagion, the device or method should use inexpensive, easily assembled parts which may replaced as necessary, which would also enable the use of interchangeable parts to adapt the scanner to different applications. The present invention solves these needs and provides further related advantages.